Stop band filter

ABSTRACT

A waveguide band-stop filter utilizes a sequence of capacitive irises to form a plurality of resonant circuits connected in tandem. In the disclosed embodiment the filter is constructed of two sets of plates having iris-defining apertures of different sizes assembled to form a laminated structure with the plates disposed in alternating sequence. The filter is dimensioned larger than the waveguide but is of relatively short length.

[ Oct. 29, 1974 Filter," MTT-ll, [963, pp. 111-116. Cohn, S. 8., DesignRelations for the Wide-Band Waveguide Filter", Pro. lre, 7-1950, pp.799-803. Feller et al., Broad-Band Waveguide Admittance Matching By Useof lrises, Pro. lre, 10-1947, pp. 1080-1085.

Primary Examiner.lames W. Lawrence Assistant Examiner-Wm. H. PunterAttorney, Agent, or Firm-Wolf, Greenfield & Sacks [57] ABSTRACT Awaveguide band-stop filter utilizes a sequence of capacitive irises toform a plurality of resonant circuits connected in tandem. In thedisclosed embodiment the filter is constructed of two sets of plateshaving United States Patent Rhodes STOP BAND FILTER [75] Inventor: JohnDavid Rhodes, Guiseley,

England [73] Assignee: Microwave Development Laboratories, NeedhamHeights, Mass.

{22] Filed: Apr. 17, 1973 [21} Appl. No.: 351,860

[52] US. 333/73 W, 333/33 [51] Int. Cl. I-l0lp 1/20 [58] Field ofSearch........ 333/73 W, 73 R, 31 A, 33

[56] References Cited UNITED STATES PATENTS 1 Claim, 6 Drawing Figuresiris-defining apertures of different sizes assembled to form a laminatedstructure with the plates disposed in alternating sequence. The filteris dimensioned larger than the waveguide but is of relatively shortlength.

N w A/ 4 H 4 a d U A 51% 3AWR n W1 3 m H// //-1 MM? 3 7 I W w M/V /HilillliVV y B3B mm U u 3 a e T 1 I m m N W r iii iuV/r//////i 1/5 H H. uR i m m d m S m w M m I N m m m m nu e iiiii mfmrm m /r iiIL i h K w m mm r w Ma L e mmha U 0 incl LCCA R g E. 3 26 H 5666 9999 TA HHHHOH 6 77 vD 3% E .5 a an? m h 2333 S STOP BAND FILTER FIELD OF THE INVENTION Thepresent invention relates in general to frequency selective apparatusfor filtering signals which are in the microwave portion of theelectromagnetic frequency spectrum. More particularly, the presentinvention relates to frequency filters for use with waveguides and ofthe band-stop type.

BACKGROUND OF THE INVENTION In many microwave systems the signalfrequency is transmitted and guided within the system with a minimum ofattenuation, while undesired frequency bands are strongly attenuated byband-stop filters which pass the desired frequencies with lowattenuation. One typical band-stop filter comprises a plurality ofband-stop resonator cavities which are coupled to the main waveguide byinductive irises. In order to avoid interaction between the fringingfields at the various resonator irises. the resonators are spacedthree-quarter guide wavelengths apart. Thus, these prior art band-stopfilters have been relatively long and bulky.

Other filters that are used for microwave frequency filtering includecorrugated and waffle-iron filters. One filter of this type is shown inU.S. Pat. No. 3,597,7l0. As indicated in that patent these filters areconstructed to fit within the dimensions of a conventional waveguide andbecause of the dimensional limitations imposed upon the filter they donot have a relatively narrow stop-band width. These types of filters areusually used as low-pass filters.

OBJECTS OF THE INVENTION One important object of the present inventionis to provide an improved band-stop filter for use with a waveguide andthat can be constructed relatively compactly.

A further object of the present invention is to provide an improvedband-stop filter in accordance with the preceding object and that isrelatively short in length in comparison with known band-stop filters.

SUMMARY OF THE INVENTION As previously indicated, the typical design fora bandstop filter involves commencing with a waveguide structure,providing spaced resonator cavities and coupling the resonator cavitiesto the waveguide by means of inductive irises. for example. Otherfilters used as band-pass filters, such as a corrugated filter, areusually constructed within the dimensional limitations of the particularwaveguide size employed. However, it has been discovered in thisinvention that a band-stop filter can be constructed by using alternateiris-defining plates ofdifferent aperture size. and by permitting one ofthe plates to have an iris comparable to the size of the waveguide andthe other plate to have an iris larger than the size of the waveguide.

In the disclosed embodiment of the invention the filter comprises afirst set of plates having iris-defining apertures of a firstrectangular size and a second set of plates having iris-definingapertures of a second rectangular size larger than the first size. Meansare provided for assembling the plates together to form a laminatedstructure with the first plates disposed alternately with respect to thesecond plates. In accordance with the invention, the first plates areconstructed having an aperture size that is comparable to the dimensionsof the waveguide. The second plates are constructed having an aperturesize that is larger than the waveguide dimensions. Regarding the secondplates the aperture in one dimension may be approximately twice thedimension of the waveguide. Finally, means are provided for coupling thefilter to opposite waveguide sections.

BRIEF DESCRIPTION OF THE DRAWINGS Numerous other objects, features andadvantages of the invention will now become apparent upon the reading ofthe following detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a perspective view of a microwave bandstop filter according toone embodiment of the present invention;

FIG. 2 is a vertical section taken along the longitudinal axis of thefilter of FIG. 1, when the filter is inserted in a waveguide structure;

FIG. 3 shows in detail the formation of resonant circuit within thefilter;

FIG. 4 represents the equivalent circuit of the resonant circuit of FIG.3;

FIG. 5 is a front elevation of a modification of the filter shown inFIGS. 1 and 2; and

FIG. 6 is a longitudinal sectional view similar to that shown in FIG. 2and modified for stopping both the second and third harmonics.

DETAILED DESCRIPTION Referring now to the drawings, and in particular toFIGS. 1-3, there is shown a microwave filter 10 which is for connectionto a rectangular waveguide section. For a signal propogated through thewaveguide struc ture, the filter causes a desired level of attenuationof components of the signal in a selected one of the harmonic bands inrelation to the main pass-band of the waveguide structure.

The filter 10 is constructed from nineteen rectangular plates, eachplate having a uniform predetermined thickness t and each plate alsohaving the same external dimensions. The plates comprise nine firstiris-defining plates ll interleaved with eight second iris-definingplates 12, and two end plates 13. Rectangular shaped apertures, theboundries of which are indicated at l4, l5 and 16, are stamped out ofthe plates 11, 12, and 13, respectively, and the plates are then coatedwith a metal having a low loss factor. The plates are secured togetherin any convenient manner to form a laminated construction. and theexternal surfaces of the assembled filter 10 are hermetically sealed bybeing coated with solder. The filter is secured in the main guide 18 ofthe waveguide structure as shown in FIG. 2.

The rectangular main guide 18 has a width a and a height I). and theapertures I4, 15 and 16 in the plates ll, 12 and 13 of the filter areeach of a width a. The height of the aperture 16 in each end plate 13 ism, as indicated in FIG. 2. As indicated in FIG. 3, the height of theaperture 14 in each first iris-defining plate 11 is c, and the height ofthe aperture 15 in each second irisdefining plate I2 is 1. In FIG.3c+2p= l. The centres of the apertures 14, 15 and 16 are co-incidentwith the centres of the plates ll, 12 and 13 in which the apertures areformed. Thus, the passage through the filt er t0 has different sectionalareas, the longi- 3 tudinal axis of the filter being coincident with thelongitudinal axis of the main guide 18.

As is more clearly shown in FIG. 3, each adjacent pair of firstiris-defining plates 11 and the second irisdefining plate 12 betweenthem together fon'n a resonant circuit indicated generally at 19 betweenthe broken lines 20. The filter has eight such resonant circuits 19 intandem with each other. Each resonant circuit reflects components ofsignals within the stop band associated with the filter, the resonantfrequency of each circuit being the centre frequency of the stop band.The centre frequency is predetermined to be such that the stop bandcomprises the selected harmonic band required to be attenuated by thefilter. The greater the number of resonant circuits within the filterthe greater the level of attenuation of components of the signal withinthe selected harmonic band.

Each end plate 13 and the first iris-defining plate 11 with which it iscontiguous comprise a transformer circuit between the main guide 18 andthe series of resonant circuits 19 of the filter 10. The height m of theaperture 16 in each end plate is arranged to be the mean value of theheight b ofthe section of the main guide 18 and the height c of theaperture 14 in each first irisdefining plate I 1. Thus, each transformercircuit has an admittance value equal to the mean of the admittancevalue of the main guide and the admittance value of each resonantcircuit 19. The shunt capacity of the discontinuity between the mainguide 18 and the adjacent resonant circuit 19 may be ignored.

The heights of the apertures in the plates can be expressed by thefollowing equation: for the end plates 13;

for the first iris-defining plaTesTh and for the second iris-definingplate 12;

l( T+ T Because K is positive m b The way in which the value of K isderived is indicated below.

Also below it is shown that, if L, is the desired level of attenuationby the filter of the components of the signal in the selected harmonicband. then the number (n) of resonant circuits 19 required in the filterto cause, at least, the desired level of attenuation is given by theequation. 2ifi0 P,+ P -'1 where P,, l 4rY/bp (Ag/Ago 1), Y is thecharacteristic admittance of a length of the filter, and (Ag) and (Ago)are respectively, the wavelength in the guide at any frequency and atthe center frequency of the stop band.

In one particular embodiment of the stop band filter for a waveguidestructure having a main passband of 5.9 to 6.4 GigaHertz, whichcorresponds to a conventional rectangular waveguide identified as a WR137 having dimensions L37 inches by .622 inches. the filter causesattenuation of components of a signal in a selected one ofthe l l8 tol2.8 GigaHertz or 17.7 to 19.2 GigaHertz etc. harmonic bands. Each plateof the filter has the same thickness. in the range 0.03 inch to 0.linch, determined by the convenience of manufacturing the plates. Theexternal dimensions of the plates are larger than the sectionaldimensions of the main guide.

The theory underlying the design of a microwave stop band filteraccording to the present invention will now be given.

In the dominant H mode for the filter, with plates 7 having rectangularshaped apertures, the equivalent circuit of the resonant circuit 19shown in H0. 3 is represented by the circuit diagram shown in FIG. 4.The equivalent circuit comprises reactance (jX) between two identicalimpedances Z, where Z is the characteristic impedance of a length of thefilter with respect to the main guide 18. This impedance Z is defined bythe transfer matrix:

icosbmjz sin 29 'Y sin 26, cos 26 (1) where (Y 1/2) and (0, B 1/2). Bbein g the signal wave velocity within the filter 10.

The reactance X is given by the equation: X 22. tan 0, where 0, lip andZ, I) 1. The transfer matrix of the resonant circuit is therefore:

maybe written as:

K565132551 it 3' Y sin 4/ cos I where iii cos (cos40 Z,Ytan0 sin46,)

ZIZZ 1-2.1 tan a, tan 201 fifth e main passband and!) are sfiiiiiifsoequation (6) becomes:

where the error factor A tan Bp/Bp In relation to the 5.9 to 6.4GigaHertz passband of the waveguide structure:

p is selected to be a quarter wavelength at the center frequency of thedesired stop band.

For stop bands far removed from the passband A is approximately L0.

if the resonant circuit is matched with the main Hence, from equation(7):

111' Z -I =0. or Z= ('1+K K where K pA/2b since Z must be less thanunity.

[jY sin nil/ cos ml:

The insertion loss L associated with the filter is given by theequation:

L [0 log [I (Z, Y,) /4 Sinmll] (ll). In the selected stop band eachseries resonant circuit is arranged, at least. to be near to resonance,and there- L l0 log [I (Z, Y,)/4 sink 2 (n cosh P)], with -.-t1 Fromequation l4 because p is always less than b and because Z isapproximately equal to L0, the contribu tion of the second term ofequation (15) is -l0db or less. Thus:

1, Li 0. 531-1: R? T1: !l. 16)

If the guide wavelengths at the lower and upper frequencies (f1) and ofthe selected harmonic band are, respectively, Ag, and Ag, then: Ag z(Ag. Mal/2 Therefore, if L is the desired level of attenuation forcomponents of the SlLfllll'l l hg SElfitEtEd hat-monk: band: LFHOI 1 TotP. .+'72a: L*1. N1 with P l 4rY/bp (Ag/Ag I) (18). For example, assumethe following dimensions:

1 .05, b/Y 0.5 and p .3l

P I 4x .05 x .05/.5 x .3l (Agl)/kg l .0s4/(x /x 1 Hence, the stopbandwidth is approximately i 6.4 percent. Note that the bandwidth is afunction of the thickness of the plates and the p dimension. Forproviding a stop band that is relatively narrow, attenuating onlyundesired frequencies, the thickness should be relatively small and thedimension p is large compared with waffle iron or corrugated filtersthereby requiring that 1 the filter have dimensions larger than theguide l8.

FIG. 5 shows a modified version of the waveguide that may be preferredin attenuating higher order modes. In this embodiment there is providedslots 21 in the first iris-defining plates ll. The slots 21 are in eachfirst iris-defining plate II and are arranged in rows which extendparallel to the longitudinal axis of the filter. The depth of the slots21 are such that the slots do not extend beyond the projection of theaperture 16 in each end plate 13 onto the plates ll.

It is possible to have a filter with a plurality of constitutent platessuch that the value of p is not constant throughout the filter, thedesign procedure being modified accordingly. The first iris-definingplate ll between adjacent resonant circuits having different values forp, and therefore different characteristic impedances 2, preferablyshould be stepped so as to have two apertures each of a thickness (r/2).The different heights of the apertures are arranged to be such that therequired different admittances for the adjacent resonant circuits areprovided. However, the first irisdefining plate ll between the resonantcircuits may have a single aperture of a constant height throughout itsthickness, the height of the aperture being such that there is obtaineda mean admittance value of the two desired admittance values for theadjacent resonant circuits.

Another embodiment of the invention is shown in FIG. 6. This embodimentis used for suppressing two stop bands.

The structure of FIG. 6 is quite similar to that shown in FIG. 2 exceptfor the addition of middle section 25. The end sections 26 may beidentical in design to the plates shown in FIG. 2. In FIG. 6 thedimension c is comparable to guide dimension b and the dimension d isapproximately equal to one-fourth b.

In order to attenuate the level of components of the signal in more thanone stopband it is also possible to combine a plurality of filters ofthe type shown in FIG. 2, different constitutent filters causing theattenuation of components of the signal in the different harmonic bands.The input and output transformer circuits of adjacent filters of such acombination comprises a single circuit having a characteristicadmittance value equal to the mean of the desired admittance values ofthe input and output transformer circuits of the adjacent filters.

A filter according to the present invention, capable of attenuatingcomponents of a signal in a selected stopband to the main passband, isparticularly advantageous when, as is usual, the passband is defined bya bandpass waveguidefilter also having a response at higher frequenciesto the main passband frequencies.

A filter according to the present invention may be employed in awaveguide structure in a communication system. In such a system it isnecessary to attenuate only components of a signal which compriseharmonics to the communication band, the communication band comprisingthe main passband of the waveguide structure. If components of thesignal in more than one harmonic band are required to be attenuated acombination of filters is employed. In such a case, conveniently, theconstitutent filters cause different levels of attenuation forcomponents in the different selected harmonic bands. Since components ofthe signal which are neither in the communication band nor in a selectedharmonic band are not required to be attenuated, each filter is simplein construction and small in size. The filter of this invention isparticularly useful when it is desired to reject only the secondharmonic.

The filter described above, with plates having rectangular shapedapertures, is for use with a main guide having a rectangular shapedsection. However. it is possible to construct a filter with plateshaving apertures of any convenient shape, and for use with a main guidehaving an appropriately shaped section.

A stop band filter according to the present invention may causeattenuation of components of a signal other than components solely in aselected harmonic band in relation to the main passband. The resonantcircuits of the filter are arranged to resonate at the predeterminedcentre frequency of the required stop band.

A filter according to the present invention, and capable of attenuatingcomponents of a signal in a predetermined stop band may not necessarilybe formed with a laminated construction from an assembly of aperturedplates, but may be constructed in any convenient manner.

The laminated construction, if employed. is simple and the designprocedure to be adopted in order to obtain a filter having a requiredspecification is simpler than with known forms of stop band filter.

A filter according to the present invention can operate successfullywith microwave signals of higher power than those required to bepropagated in a communication system. The filter also has a lowerinsertion loss factor in relation to the main passband than for somepreviously known constructions of such filters.

Having described a limited number of embodiments of this invention, oneskilled in the art should readily recognize that other structures willfall within the spirit and scope of the present invention which is to belim ited solely by the appended claims.

What is claimed is: l. A narrow band band-stop filter for use with awaveguide having dimensions a and b and comprising: a first set ofrelatively thin plates respectively defining a first set ofiris-defining apertures having an aperture dimension c, a second set ofrelatively thin plates respectively defining a second set ofiris-defining apertures having an aperture dimension 1, said aperturedimension 1 being larger than said aperture dimension c and larger thanthe waveguide dimension b, said first and second sets of plates allbeing of approximately the same individual thickness 1 and beingdisposed in alternating sequence forming corrugations; an aperturedifference dimension p being defined as:

p l-c/Z wherein the stop bandwidth is a function of the ratio:

4tlbp and the ratio tlp is caused to be made small by constructing theplates on the order of 0.1 inches thick or smaller and making the pdimension on the order of a quarter wavelength at the center frequencyof the stop band,

and means coupling the filter to separate waveguide sections.

t 3! i i l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 334 422 Dated Oct. 29, l974 Inv nwfl John David Rhodes It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 2, line 64, should read as follows --defining plate 12 is L InFIG. 3 c 2p i1.

Column 3, line 41, should read --t=b[(1+| -K]+2 Column 4, line 45,equation 6 should read as follows 1 Y tan 62 cot 2e 1/2 1 l-'Z ;Y tan 62tan 29 In column 5, the demominator of equation l2 should read (l-Ago AgCollumn 5, line 33, the equation should read --P=l+t Y[bp( -g/Ag 1Column 5, line 34 should read -since P Column 5, line 57 should read--Po=l-4x.05x.05/.5x.3l(xglxgo-l) Claim 1 in lines 8 and 9, change"dimension 1 to --dimension 2...

Claim l line 19, should read --p=(-c)/2 Signed and sealed this 22nd dayof April 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks FORM PO-1050 (\0-69) uacoMM no 00310 P69 U-S. GDVIINIINTPRINTING OF'ICI ll. O-SlO-SSI.

1. A narrow band band-stop filter for use with a waveguide havingdimensions a and b and comprising: a first set of relatively thin platesrespectively defining a first set of iris-defining apertures having anaperture dimension c, a second set of relatively thin platesrespectively defining a second set of iris-defining apertures having anaperture dimension 1, said aperture dimension 1 being larger than saidaperture dimension c and larger than the waveguide dimension b, saidfirst and second sets of plates all being of approximately the sameindividual thickness t and being disposed in alternating sequenceforming corrugations; an aperture difference dimension p being definedas: p 1- c/2 wherein the stop bandwidth is a function of the ratio:4t2/bp and the ratio t2/p is caused to be made small by constructing theplates on the order of 0.1 inches thick or smaller and making the pdimension on the order of a quarter wavelength at the center frequencyof the stop band, and means coupling the filter to separate waveguidesections.